1. Field of the Invention
This invention relates to an improved method of processing high level waste in order to separate the actinides and fission products, and more specifically, this invention relates to an improved method of reprocessing spent nuclear fuel providing improved separation of minor actinides from lanthanides on an industrial scale.
2. Background of the Invention
The recent renewed interest in nuclear power stems from higher petroleum costs and also petroleum role in carbon dioxide emissions. According to the Intergovernmental Panel on Climate Change (IPCC), total carbon emissions from the energy sector are expected to grow from today's 6.5 billion tons to 13 billion tons in 2050, with total cumulative emissions of carbon through 2050 of 440 billion tons.
The management of a nuclear system of a scope to even begin ameliorating this anticipated carbon load requires planning. For example, a worldwide capacity of 3500 GWe (a figure of illustrative convenience, ten times current capacity), if based on a once-through fuel cycle using light water reactors, would generate roughly 700 tons of plutonium annually, and would require on the order of one-half million tons of natural uranium annually. If based on liquid-metal plutonium breeder reactors, it would involve the fabrication into fresh fuel annually of over five thousand tons of plutonium. With the legacy of light water reactors in existence, the reprocessing of spent fuel from these reactors will be an ongoing concern for the foreseeable future.
The disposition of the waste which results from the reprocessing of irradiated nuclear power reactor fuel elements containing highly radioactive waste generated from reprocessing of spent nuclear fuel contains various elements which retain their toxicity over long periods of time is one of the major problems facing the nuclear power industry today. One approach is to solidify the liquid wasted as it come from there reprocessing facility into a stable solid material which can be stored in the earth for a period of time sufficient for the radiation to decay to sage levels. Alternatively, more toxic elements can be removed by appropriate process to remove the more toxic elements, leaving lower toxicity material which easier to store or treat. Elements such as transuranium elements (TRU), and the treatment and disposal of these elements present a problem. Of the transuranium elements, the minor actinides, americium (Am) and curium (Cm) have especially high toxicity, so it is desirable to remove them from radioactive waste and to deal with them appropriately. When using Am and Cm as fuel for their transmutation, Am, Cm and rare earth elements which chemically resemble each other, have to be separated. A number of separation processes have been developed to separate toxic fuel waste to permit more efficient handling and storage.
Ninety percent of the waste proposed for disposal at the geologic repositories generally consists of spent nuclear fuel, such as that generated by commercial nuclear power plants, government reactors, and naval propulsion plant reactors. The remaining ten percent of wastes proposed for disposal at Yucca Mountain consists of high-level radioactive waste, which is produced mainly from spent nuclear fuel reprocessing, such as PUREX, discussed below. Storage of high level waste without any attempts at heat load and/or volume reduction can quickly deplete the space allowed for such waste.
Briefly, the PUREX process consists of a sequence of chemical process steps comprising initially treating the waste of scrap material or spent fuel containing uranium compounds with an aqueous solution of nitric acid (HNO3), and thereby dissolving the uranium to produce uranyl nitrate (UO2 (NO3)2), neptunium nitrate NpO2NO3 and plutonium nitrate Pu (NO3)4 (Fission Products, FP) and other acid soluble components within an aqueous phase. This aqueous phase containing the acid dissolved components including uranyl nitrate, and any acid insoluble components of the waste is passed down through an extraction column, pulsed columns or mixer-settlers while an organic phase of tri-butyl phosphate in an organic diluents of paraffinic oil, such as kerosene, is passed up through the extraction column in counter-current flow with the aqueous phase. The soluble uranium compounds comprising uranyl nitrate of the aqueous phase are extracted therefrom by the organic phase and combined with the tri-butyl phosphate. This separates the uranium and carries it within the organic phase from the extraction column. The aqueous phase and the organic phase each exit from the extraction column at opposite ends from each other and from their respective entries, the aqueous phase with the acid soluble raffinate contaminants and the organic phase with the separated uranium, neptunium and plutonium. The raffinate produced from the PUREX process contains, generally, Fission Products (FP-transition elements such as zirconium, molybdenum, technetium, including noble metals as ruthenium, rhodium, palladium, platinum, rear earth elements—lanthanum, cerium, praseodymium, neodymium promethium samarium, europium; and actinides-protactinium, americium (minor actinide), curium (minor actinide), and trace amounts of plutonium, uranium, gadolinium and terbium.
The organic phase effluent from the extraction column or the bank of centrifugal contactors carrying separated uranium compounds is then passed up through a stripping column while water is passed down through the stripping column in counter-current flow with the organic phase. The water releases the uranium from the tri-butyl phosphate of the organic phase whereby it is transferred to and carried within the aqueous phase. The aqueous phase and the organic phase each exit from the stripping column at opposite ends of the separator from each other and from their respective entries, the organic phase containing the uranium and plutonium compounds is treated to separate uranium and plutonium for recovery from the contaminants. The organic phase is then recycled back through the extraction column. Typically, the procedure is carried out with a continuous flow of all components through the system comprising the extraction column and stripping column.
The desired product of the PUREX solvent extraction process is a high purity aqueous phase effluent from the system containing virtually all the uranium of the initial waste fed into the system. However, some losses of uranium occur in the raffinate effluent by design and represent an economic loss. There is an acknowledged “trade-off” between the uranium product purity obtainable and the level of uranium loss in the raffinate. The extent of this balance of benefits depends substantially upon individual design. To enhance impurity reduction, some system designs include an intermediate scrub-section adjoining or as a section of the extraction column. However, the PUREX process does not recover other components of the spent fuel rods such as americium, cesium, strontium, neptunium, and technetium. Thus, PUREX produces high-level waste primarily comprised of transuranic elements and fission products. Improvements on the PUREX process have been developed to correct the removal of selected elements. Similar processes, such as but not limited to, COEX, and AREVA processes, such as DIAMEX, treatment process produce a nitric acid based aqueous raffinate stream, similar to the PUREX raffinate. As discussed previously, it is desirable to separate minor actinides, in particular Am and Cm, from the lanthanides to minimize long term storage volume.
As part of the management of minor actinides it has been proposed that the lanthanides and trivalent minor actinides should be removed from the PUREX raffinate by a process such as TRUEX or DIAMEX. In order to allow the actinides such as americium to be either reused in industrial sources or used as fuel, the lanthanides must be removed. The lanthanides have large neutron cross sections and hence they would poison a neutron driven nuclear reaction. Other systems such as the dithiophosphinic acids are being worked on by some other workers.
One method of separating transuranium elements including trivalent actinides such as Am or Cm and nuclear fission products (FP) from highly radioactive waste, is the TRUEX (TRansUranic EXtraction) method. In the TRUEX method, octyl(phenyl)-N,N-di-isobutylcarbamoylmethylphosphine oxide (referred to hereafter as CMPO) and tributylphosphate (referred to hereafter as TBP) are mixed with a hydrocarbon diluent (e.g. n-dodecane) to make a solvent with which transuranium elements are extracted. This solvent will be referred to hereafter as a CMPO-TBP mixed solvent. The CMPO-TBP solvent is brought into contact with acidic radioactive waste to separate transuranium elements and nuclear fission products. According to the TRUEX method, transuranium elements including trivalent actinides such as Am and Clare extracted by the CMPO-TBP mixed solvent, leaving nuclear fission products in the aqueous phase.
However, rare earth elements in the nuclear fission products are also extracted together with transuranium elements by the CMPO-TBP solvent. Consequently, the method does not work well to separate trivalent actinides such as Am and Cm in the transuranium elements from rare earth elements.
The DIAMEX (DIAMideEXtraction) process has the advantage of avoiding the formation of organic waste which contains elements other than carbon, hydrogen, nitrogen and oxygen. Such an organic waste can be burned without the formation of acidic gases which could contribute to acid rain. The DIAMEX process is being worked on in Europe, primarily through the French nuclear program. The process is sufficiently mature that an industrial plant could be constructed with the existing knowledge of the process. In common with PUREX this process operates by a solvation mechanism.
The TALSPEAK (Trivalient Actinide Lanthanide Separation by Phosphorous reagent Extraction from Aqueous (K) Complexes) process uses an acidic organophosphorus reagent (HDEHP, di-2-ethylhexyl-phosphoric acid) and an aminopolyacetic type complexing agent (e.g. diethylenetriamine pentacetic acid) to separate trivalent actinides and rare earth elements. According to this TALSPEAK method, trivalent actinides and rare earth elements can be separated from each other with high efficiency. However, according to the aforesaid TALSPEAK method, a pH of approximately 3 must be maintained in the separating step in order to obtain suitable separation conditions.
The highly radioactive waste generated by reprocessing of spent nuclear fuel normally contains acid of approximately 3M concentration. It was therefore necessary to subject the highly acid waste to a denitrification step as a pretreatment to reduce its acidity before using the TALSPEAK method. It was also necessary to maintain the pH at 3 throughout the entire separation process, and normally, the pH had to be controlled by adding pH buffers such as highly concentrated carboxylic acids (e.g. lactic acid) to the stripping solution. It was difficult to adjust the pH precisely.
The UREX (URanium EXtraction) process is a modification of the PUREX process to prevent the plutonium from being extracted. This can be done by adding a plutonium reductant before the first metal extraction step. In the UREX process, ˜99.9% of the uranium and >95% of technetium are separated from each other and the other fission products and actinides. The key is the addition of acetohydroxamic acid (AHA) to the extraction and scrub sections of the process. The addition of AHA greatly diminishes the extractability of plutonium and neptunium, providing greater proliferation resistance than with the plutonium extraction stage of the PUREX process. Additional modifications of the UREX were developed to improve the separation of specific elements. UREX produces an intermediate raffinate stream of similar composition to the PUREX raffinate stream.
A separation factor (SF) may be defined which measure the process capability to separate elements; the higher the separation factor rating for a given set of element the more capable the process in separating the components. The factor is calculated from the ratio of elements, such as Europium Eu and Americium Am. TALSPEAK are rated in the order of a Eu/Am separation factor of from about 50 to about 100.
The process columns are typically agitated by either pulse pumps or reciprocating plates to permit optimal droplet formation and coalescence on each plate. This agitation is most commonly referred to as mixing energy. Mixing energy is critical to efficiency of the extraction column and helps establish a characteristic uranium profile. Excessive mixing energy or flow rates can cause flooding, a condition which precludes flow of one or both liquid operating mode phases in the column. The term flooding refers to a condition in which the two immiscible phases flow countercurrent past each other with a relative velocity that is sufficient to impede the steady flow of one phase or the other phase. In the PUREX process the bulk of the impurity removal or decontamination of uranium compounds is achieved near the inlet of the extraction column for feeding the acid treated waste material. The most efficient operation of the extraction column is substantially at the level of flooding which produces the maximum removal.
When the extraction column is operating at a steady state, a uranium concentration profile therein can be obtained by sampling either the organic or aqueous phase at several points along the vertical length of the column. The profile depends on the degree of trade off chosen between uranium product purity and level of uranium loss.
U.S. Pat. No. 5,708,958 awarded to Koma, et al. on Jan. 13, 1998 discloses a method is provided for separating trivalent actinides and rare earth elements in the TRUEX method using a CMPO-TBP mixed solvent. The method of separating trivalent actinides and rare earth elements comprises a trivalent actinide/rare earth extraction step wherein trivalent actinides and rare earth elements are extracted by a solvent from highly acid waste generated by reprocessing of spent nuclear fuel, a nitric acid removal step wherein the nitric acid concentration of the solvent used to extract the trivalent actinides and rare earth elements is reduced, and a separation step wherein the trivalent actinides and rare earth elements contained in the solvent of low nitric acid concentration, are separated from each other. U.S. Pat. No. 5,256,383 awarded to Cordier, et al. on Oct. 26, 1993 discloses a process for the separation of actinides from lanthanides by the selective extraction of the actinides in an organic solvent incorporating a propane diamide. This process consists of adding to the aqueous nitric solution containing the actinides and lanthanides a thiocyanate, e.g. ammonium thiocyanate, followed by the contacting of said solution with an organic solvent incorporating at least one propane diamide, e.g. 2-tetradecyl-N,N′-dimethyl-N,N′-dibutyl-propane diamide and optionally a quaternary ammonium salt such as trilauryl methyl ammonium (TMA) thiocyanate or a mixture of quaternary ammonium thiocyanates. This leads to distribution coefficients DM for the actinides (Am) and the lanthanides (Eu, Ce) making it possible to achieve high actinide/lanthanide separation factors.
None of the aforementioned patents or articles discloses a method which can achieve the objectives of high separation efficiency in separating minor actinides from lanthanides while minimizing process steps. Also, none of the aforementioned patents or articles discloses a method to produce feed streams suitable for detailed processing of spent nuclear fuels so as to separate transuranics and lanthanides.
A need exists in the art for spent fuel reprocessing system that minimizes the number of processes steps. The processing method should provide for the separation of actinides and lanthanides so the actinides can be used as nuclear fuels. The process should further result in a significant reduction in heat, radiotoxicity, or volume of high level waste. The process should minimize the use of separation problems/iteration effects brought about by the use of CMPO produce feeds suitable for nuclear fuel and/or targets for transmutation of transuranic elements.